In every switching exchange, be it numerical (i.e. digital) or not, there are always two subsystems present:
the set of line interfaces communicating with subscribers and trunks; and
the automatic switching installation.
The first subsystem serves for adapting the line criteria to those of the switching installation while the second serves to provide temporary connection between two lines, on the basis of the processing results of the signaling information.
Exchanges of the type designed to switch numerical signals, in which the automatic switching installation is made up of a certain number of centralized units, are well known: these have a connection field designed to carry out the switching of the numerical signals on the basis of the processing of the signaling information, which is carried out by a centralized control unit.
In order to avoid inefficiency of the switching exchange due to a breakdown in one of the centralized units, these are usually duplicated; consequently, means designed to regulate the formalities of their operation "in duplicate" must be provided.
The cost of each line connected to a switching exchange of the type described is linked to that of the aforesaid centralized units and varies in relation to the capacity of the exchange (understood as the maximum traffic-handling capacity) and the number of lines actually connected.
If the switching exchange is designed to handle an amount of traffic that is not much greater than that of the lines connected to the exchange, the cost per line is not very high, but there is the inconvenience that, for a user increase exceeding the exchange capacity, it is necessary to replace the previously installed centralized units with others of greater capacity, on account of the fact that these units are not modular and therefore do not allow capacity expansion.
On the other hand, if a switching exchange with a much larger capacity than the number of lines to be connected is installed, there is no longer a saturation problem, but the cost per line is extremely high and therefore the exchange is uneconomical. Recently, in order to lower the effect of the cost of the centralized part upon the cost of the single terminations, exchanges made up of switching modules of reduced dimensions, connected by means of group stages and/or trunks, have been designed. Solutions using centralized or distributed controls but still with a hierarchial structure and duplicated elements inside the modules are well known.
Routing, in all cases, is deterministic and requires that several control units or every module be informed of the distribution of the terminals and the current status of the field of connection.